Catalyst 3750 QoS Notes

Refer to the following document for an excellent reference on Catalyst 3750 Qos:
https://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml

Priority Queuing

• Inbound priority queue: Q2 (of two queues)
• Outbound priority queue: Q1 (of four queues)

Normal Behavior with “mls qos” Enabled

• CoS is found in the 802.1p priority bits.  When entering the switch, these incoming incoming frames are marked to CoS 0.
• CoS 0 is referenced against the cos-to-dscp mappings configured on the switch.  By default, CoS 0 is mapped to DSCP 0.
• The frame is passed on through the switch with CoS 0 / DSCP 0

Three Options for CoS/DSCP Classification and Marking

• There are three methods used for classification and marking on Catalyst 3750 switches:

• Port-based configuration using the “mls qos” interface commands.
• MQC-based configuration using class-maps and policy-maps.
• VLAN-based configuration

Port-Based Methods

• Access Port (i.e. no Cisco IP phone connected).  Trust DSCP because there is not dot1q/ISL tag in the frames, which is where the CoS bits are stored.

interface FastEthernet 1/0/10
mls qos trust dscp

• Trust Port (i.e. Cisco phone).  The Cisco phone uses 802.1p frames for voice vlan traffic.  PC traffic is sent untagged, which pass through as DSCP 0 regardless of the trust state.

interface FastEthernet 1/0/20
mls qos trust cos

• Uplink Port (i.e. Router-facing)

interface FastEthernet 1/0/1
mls qos trust dscp

Additional Port-Based Commands

• “mls qos cos 3 override” – Sets tagged and untagged frames to the value defined.  This overrides any “mls qos trust [cos | dscp]” values.
• “switchport priority extend cos 0” – Configures the phone to reset PC-egressing traffic to CoS 0
• “mls qos trust device cisco-phone” – Trusts the CoS markings made by the Cisco IP phone.  It also prevents someone from plugging a PC directly into this switchport (i.e. not daisy-chaining through the phone’’s built-in switch) and receiving any preferential marking.

MQC-Based Classification and Marking Example

HQ-3750(config-std-nacl)#ip access-list extended ACL-VOICE-SIGNALING
HQ-3750(config-ext-nacl)#permit tcp any any eq 1720
HQ-3750(config-ext-nacl)#permit tcp any any eq 2000
HQ-3750(config-ext-nacl)#permit tcp any any eq 2428
HQ-3750(config-ext-nacl)#permit tcp any any eq 5060
HQ-3750(config-ext-nacl)#permit tcp any any range 11000 11999
HQ-3750(config-ext-nacl)#permit udp any any eq 1718
HQ-3750(config-ext-nacl)#permit udp any any eq 1719
HQ-3750(config-ext-nacl)#permit udp any any eq 2427
HQ-3750(config-ext-nacl)#permit udp any any eq 2428
HQ-3750(config-ext-nacl)#permit udp any any eq 5060
HQ-3750(config-ext-nacl)#exit

HQ-3750(config-std-nacl)#ip access-list extended ACL-VOICE-MEDIA
HQ-3750(config-ext-nacl)#permit udp any any range 16385 32767

HQ-3750(config)#class-map CLASS-VOICE-SIGNALING
HQ-3750(config-cmap)#match access-group name ACL-VOICE-SIGNALING
HQ-3750(config-cmap)#exit
HQ-3750(config)#class-map  CLASS-VOICE-MEDIA
HQ-3750(config-cmap)#match access-group name ACL-VOICE-MEDIA
HQ-3750(config-cmap)#exit

HQ-3750(config)#policy-map POLICY-VOICE
HQ-3750(config-pmap)#class CLASS-VOICE-SIGNALING
HQ-3750(config-pmap-c)#trust cos
HQ-3750(config-pmap-c)#exit
HQ-3750(config-pmap)#class CLASS-VOICE-MEDIA
HQ-3750(config-pmap-c)#set dscp af26
HQ-3750(config-pmap-c)#exit
HQ-3750(config-pmap)#exit

HQ-3750(config)#interface gigabitEthernet 1/0/13
HQ-3750(config-if)#switchport access vlan 10
HQ-3750(config-if)#switchport mode access
HQ-3750(config-if)#switchport voice vlan 100
HQ-3750(config-if)#spanning-tree portfast
HQ-3750(config-if)#service-policy input POLICY-VOICE
HQ-3750(config-if)#exit

Default Scheduler Configuration

The priority queue is disabled. Both the shaped and shared mode are configured for the SRR. Shaped mode weights override the shared mode value. Therefore, the net result is queue 1 is serviced in shaped mode and queues 2, 3, and 4 are serviced in shared mode. This means queue 1 is serviced with an absolute value that is (1/25) percent, or four percent, of the bandwidth. Queues 2, 3 and 4 are serviced at 25 percent of the bandwidth. If the bandwidth is available, then queues 2, 3 and 4 can be serviced at more than 25 percent of the bandwidth.

Distribution1#show mls qos int gigabitEthernet 1/0/20 queueing
GigabitEthernet1/0/20
Egress Priority Queue : disabled
Shaped queue weights (absolute) :  25 0 0 0
Shared queue weights  :  25 25 25 25
The port bandwidth limit : 100  (Operational Bandwidth:100.0)
The port is mapped to qset : 1